Thank you for your help. In fact, first, I tried to directly include i2c.h, but there were a lot of errors. After having defined __KERNEL__ the errors disapeared. But gcc was not able to find the function "i2c_master_send" which is located in drivers/i2c/i2c-core.c.
I made the Kernel (linux-2.6.9-rc1-gum) with I2Csupport which generated i2c-core.o but I still could not compile the program. I made uClibc and then buildroot but I think a forgot something because it still doesn't work.
Could you quickly summerize the steps that have to bo done to use i2c on the gumstix ?
Thanks a lot